1. Field of the Invention
This invention relates to a system and method for the supporting of underground mine faces, bunkers and similar cavities in mining, in particular bituminous coal mining.
2. Description of the Prior Art
In underground coal mining, the coal is extracted in the seam by a process which is called stoping. By means of modern extraction equipment, the seam is mined over a width of several hundred meters.
Shearer loaders and coal planers are conventional pieces of extraction equipment. The shearer loader is an oversized milling tool device. Shearer loaders and planers move back and forth along the mining face and shear or plane the coal out of the seam. The mining equipment consequently advances at right angles to the mining face.
The path for the extraction equipment is kept free with a support for supporting the roof. The roof is defined as the rock above the extraction equipment. Conventionally, the support of the extraction equipment, which is being used, is done in steps. For this reason, the support is called a walking support.
In the mining direction behind the support, the roof is no longer supported from below. Therefore, the rock or roof breaks off if particular measures are not taken to fill up the cavity which is formed. The cavity running along the front being mined and protected by the support is called the face.
By means of suitable conveyors, the coal extracted at the face is transported away from the face. The face empties into a so-called gallery. The galleries are defined as the underground path from the face--i.e. from the point where the coal is mined--to the shaft or the bunker. While the face is constantly moving, the galleries remain fixed for a rather long period of time. There are galleries which have remained in the same place for decades. As a rule, the latter are galleries which are used simultaneously for several mining areas or seams.
Galleries which are not as permanent are constructed merely to accompany a face become unimportant as soon as the mining of the face is completed.
The galleries accompanying the face can be constructed along with the face, i.e. the galleries are driven forward at approximately the same rate as the mining. The galleries can also be constructed in advance of the mining.
As soon as the galleries have lost their importance, the galleries are struck. Striking is the term used for the removal of the support in the galleries. In many galleries which accompany the face, the support is struck as the mining of the face proceeds.
As at the face, support is also necessary in the galleries to support the rock overhead. In earlier times, all supports were rigid. That was true both for wooden and for steel supports. Rigid supports have the disadvantage that they are not flexible. For these supports, there are only two alternatives: stand or break. That was a major disadvantage, because frequently peak loads occur in the rock which exceed the stability of the support.
The yielding support was developed many years ago. Yielding supports represented major progress in longwall mining. The idea behind yielding supports was to take advantage of a doming effect in the rock. The doming effect is the ability of the rock to be self-supporting, either in whole or in part. Connected with yielding support was the knowledge that rock movements can be damped by means of a flexible support. The flexible support yields under peak loads, until the peak loads are reduced by the formation of a new dome.
The use of steel profiles represented another step forward in yielding supports. Generally, steel profiles suitable for such an application have the shape of a channel. In particular, yielding support is made possible by the fact that the arch support or, with a closed support, the support ring is made up of individual parts. Generally, an arch support has at least three parts. The individual channels of the arch support overlap one another. In the overlapping region, the channels are pressed together by means of connecting straps with such strength that the friction in the overlapping region is stable under the normal rock pressure, but yields under peak loads. During yielding, the channels are displaced into one another in the direction of the arch.
The dimensions of the support profiles increase with increasing depth. That can be explained easily by the rock pressure which increases with increasing depth. In other words, the support profiles must have a higher moment of resistance corresponding to the increased rock pressure. Moreover, the interval between arch supports decreases with increasing depth. That can also be explained by the increasing rock pressure. In other words, the open space between the individual arch supports, in which the roof is not supported, constantly decreases as the depth increases.
For decades, the open space between the arch supports has been protected with wire mats. Again and again--even at shallow depths--rock strata peel off or release rocks between the individual arch supports. Not only does that present an extreme danger for the miners, but it is also disadvantageous for the support system. Against this background, attempts have been made to protect the space between the arches with the mats.
Such protection systems have a more or less long life span and, depending on the stability of the rock, the protection can be damaged even after a relatively short time. Such damage requires expensive repairs. In the context of this repair work, the space between the arch supports is protected by additional wire mats. Protection can also be achieved by means of a suitable gunite lining, which can be worked into a concrete shell between the arch supports.
The prior art also includes the initial application of a gunite lining to the rock eruption or surface or interface, followed by the installation of the yielding support in the cemented gallery.
With the mats and the gunite, the arch supports in themselves form a more or less closed support. Against this background, it became possible to consider a closed support made of steel. Such considerations were disclosed, for example, in German Patent Publication Published for Opposition Purposes No. 27 02 672, which is incorporated by reference herein. The proposal of the prior art disclosed therein combines an inner steel shell with backfilling. Backfilling in this case includes the application of mortar in the space between the inner steel shell and the rock. However, the solutions of the prior art have the disadvantage that they have not been used so far in actual practice. In the type of support described above, that may be due to the fact that the external concrete shell breaks under the action of a rock movement and the resulting excess load on the support. Then, all that is left is the modest residual strength of the inner steel shell. Recently, no one has grappled with the question of a closed steel support for galleries. This fact is all the more remarkable since, in various mining regions, work is being conducted at increasing depths, and the above-mentioned support problems have become more critical.